1. Field of the Invention
The present invention relates to a low defect density, substantially crack-free (Ga,Al,In)N material, useful for fabrication of Group III-V nitride devices, and also relates to a hydride vapor phase epitaxy (HVPE) process for making such low defect density, substantially crack-free (Ga,Al,In)N material.
2. Description of the Related Art
(Ga,Al,In)N materials have long been considered for potential device applications, including UV-to-green light emitting diodes, lasers and detectors, as well as various other high temperature, high power, and/or high frequency electronic devices. The appeal of (Ga,Al,In)N materials for such applications is that the band gap of such material can be adjusted by correspondingly varying the composition, to yield band gap values in the range of from 1.9 to 6.2 electron volts (eV).
The potential of GaN and related materials associated with such wide energy bandgap enables a myriad of devices ranging from ultraviolet laser diodes to solar blind detectors. The key to realizing this potential is fabricating high quality material. The use of high quality lattice-matched native nitride substates would be an ideal template on which to fabricate such high quality material. Unfortunately, a suitable, high quality lattice-matched substrate for (Ga,Al,In)N does not exist. As a result, poor lattice-match materials such as sapphire have been used as a substrate in prior art attempts to grow suitable (Ga,Al,In)N layers for device fabrication. Due to the lattice mismatch, the (Ga,Al,In)N layers grown on sapphire or related materials are characterized by a large defect density. The art has proposed the use of buffer layers to compensate for the lattice-mismatch, but such approach has not been satisfactory in yielding useful base structures for device fabrication.
The majority of the defects in heteroepitaxially grown GaN are threading dislocations (TDs) which are associated with the misfit between the GaN and substrate. GaN grown on sapphire, silicon carbide or other similarly poorly lattice-matched substrate, typically contains greater than 108 dislocations per cm2 of surface. The dislocations form to accommodate the difference in lattice constant between the substrate and GaN material grown on the substrate. Defects/dislocations generated in the initial layer propagate to the active region of the device. In addition, similar dislocations, although lower in density, occur because of lattice constant differences between the individual layers of the device.
Although dislocations have long been known to be a serious problem for conventional Group III-V devices, only recently has there been direct evidence that dislocations are associated with undesirable materials characteristics and device problems in the III-V nitride system. A correlation exists between mixed character threading dislocations and non-radiative recombination centers and when the density of dislocations is greater than ˜109 cm−2, negatively charged dislocations can be the dominant scattering source in GaN. Further, compositional and growth rate inhomogeneities, including V-defects, originate at the site of threading dislocations and, as may be shown by Electron Beam Induced Current (EBIC) investigation, dislocations may also serve as a path for Mg migration.
As an example of such defect-impacted performance character, for a (Ga,Al,In)N-based ultraviolet laser diode as an illustrative device structure, more than 2 orders of magnitude increase in the lifetime of the ultraviolet laser diode may be achieved by suitable formation of the GaN film material with a substantially reduced density of defects in the material.
Thus, the large lattice constant and thermal coefficient of expansion (TCE) mismatch between the epitaxial films and foreign substrates results in a high density of optically and electrically active defects which limit device performance and lifetimes. There is therefore a compelling need to lower the defect density in (Ga,Al,In)N materials.
With respect to the (Ga, Al, In)N material of the present invention as hereinafter described, the art generally has taught away from the process conditions for growth of (Ga, Al, In)nitride materials that are employed in the practice of the present invention. The art has, therefore, not achieved a dislocation density less than 107 cm−2 for crack-free areas larger than 1 cm2. A summary of the relevant teachings of the art is therefore set out below, by which the advance and achievement of the present invention may be better appreciated. Perkins et al. (N. R. Perkins, M. N. Horton, Z. Z. Bandic, T. C. McGill, and T. F. Kuech, Mat. Res. Soc. Symp. Proc. 395 (1996) 243) describes the effect of growth temperature on the crystallinity of a GaN film produced by HVPE and the use of growth temperatures in the range of 1030° C. to 1050° C. Such growth conditions are disclosed to minimize the full width half maximum (FWHM) of double crystal x-ray diffraction peaks over the temperature range of 850° C. to 1100° C. At the same time, the width of the photoluminescence excitation was not improved by using lower temperatures in this study. Perkins et al. recognized that “under non-optimum growth conditions, occasional pits are noted along the surface . . . [that] vary in size and distribution,” and they observed cracking in films that were greater than 20 μm in thickness.
Molnar et al. (R. J. Molnar, W. Gotz, L. T. Romano, N. M. Johnson, J. Cryst. Growth, 178 (1997) 147.) disclose that higher growth temperature and slower growth rates flattened out hexagonal islands, (i.e., produce smoother surface morphology).
Hwang et al. (J. S. Hwang, A. V. Kuznetsov, S. S. Lee, H. S. Kim, J. G. Choi, and P. J. Chong, J. Cryst. Growth, 142 (1994) 5) describe surface morphology improvements with increased growth temperature.
Nickl, et al. (J. J. Nickl, W. Just, and R. Bertinger, Mat. Res. Bull. 9 (1974) 1413) disclose that a growth temperature of 1030° C. optimized the ratio of near band edge photoluminescence emission (by a greater extent in the case of higher quality films) to deep level emission at ˜2.2 eV.
Melnik et al., (Y. V. Melnik et al., MRS Internet Journal, 2 (1997) article 39) grew GaN on SiC substrates and etched away the SiC by reactive ion etching. The GaN dimensions were at most 7 mm per side, limited by TCE-related cracking.
Poroswski, et al. (S. Poroswski, et al., Mat. Res. Soc. Symp. Proc. 449 (1997) 35) report free-standing GaN as large as 0.7 cm2 by high pressure sublimation.
Usui, et al. (A. Usui, et al., Jpn. J. Appl. Phys. 36 (1997) 899) describe low defect density thick HVPE GaN epitaxial growth producing a defect density of 6×107 defects/cm2 at growth rates of up to 100 microns per hour. The GaN was grown on SiO2 patterned substrates.
Romano, et al. (L. T. Romano, B. S. Krusor, and R. J. Molnar, Appl. Phys. Lett. 71 (1997) 2283) describes formation of GaN material with a defect density of 5×107 cm−2 at the upper surface of the material and discloses that “many of the threading dislocations are not perpendicular to the surface; therefore, dislocation reactions can occur with increasing film thickness.”
Thus, the art generally has taught away from the process conditions for growth of (Ga, Al, In)nitride materials that are employed in the practice of the present invention. The art also has not achieved a dislocation density less than 107 cm−2 for crack-free areas larger than 1 cm2.
The art teaches the use of a two step growth process for the growth of GaN-based materials. Relative to the two step process of the present invention hereinafter more fully described, the first step in prior art processes is, however, carried out at substantially lower temperature (400-600° C.), the first step growth layer is much thinner (<200 nm in all cases), and the role of the first step in such prior art processes is to promote uniform surface coverage or nucleation on the substrate. Defect density less than 108 cm−2 have not been achieved using this method. The first layer in the prior art processes is amorphous or highly defective.
U.S. Pat. No. 5,563,422 issued Oct. 8, 1996 to S. Nakamura et al. describes a gallium nitride-based III-V compound semiconductor device having a gallium nitride-based III-V compound semiconductor layer on a substrate, and an ohmic electrode provided in contact with the semiconductor layer. The substrate may be sapphire or other electrically insulating substrate.
In the Nakamura et al. patent, the GaN layer provided on the substrate is formed by MOCVD. In practice of the teachings of the Nakamura et al. patent, the GaN nucleation layer thickness achievable is only on the order of 100 Å. The defect density in the nucleation layers is thought to be high, perhaps on the order of 1E11 cm−2 or greater, and yields a defect density of 1E9 to 1E10 cm−2 or greater in the subsequently grown GaN layer.
U.S. Pat. No. 5,385,562 issued Jan. 31, 1995 to T. D. Moustakas describes a method of preparing highly insulating GaN single crystal films by MBE by a two step growth process that includes a low temperature nucleation step and a high temperature growth step. The low temperature nucleation process is carried out at 100-400° C., resulting in an amorphous GaN nucleation layer of thickness between 200 and 500 Å. The defect density in the nucleation layer is 1E11 cm−2 or greater, and yields a defect density of 1E9 to 1E10 cm−2 or greater in the subsequently grown GaN layer.
Japanese Patent 60-256806 to Akasaki et al. describes an AlN nucleation step, grown at 600° C. to thicknesses of approximately 50 nm by MOCVD. Similarly, the defect density in the nucleation layer is thought to be high, e.g., 1E11 cm−2 or greater, and yields a defect density of 1E9 to 1E10 cm−2 or greater in the subsequently grown GaN layer.
It would be a substantial advance in the art to utilize substrates such as sapphire, on which (Ga,Al,In)N could be reliably and reproducibly grown with a substantially reduced defect density, as compared for example to the defect levels which are typically achieved by growth of epitaxial layers of GaN on sapphire, by molecular beam epitaxy (MBE), metal-organic vapor phase epitaxy (MOVPE), and prior art hydride vapor phase epitaxy (HVPE).
It therefore is an object of the present invention to provide a low defect density (Ga,Al,In)N material and a process for making same.
It is another object of the invention to provide a low defect density, substantially crack-free (Ga, Al, In)N material which is homogeneous over a large area.
It is still another object of the invention to provide (Ga,Al,In)N material on lattice-mismatched materials such as sapphire, which has a substantially reduced level of defects relative to (Ga,Al,In)N produced by the prior art.
It is another object of the invention to provide a low defect density, large area, substantially crack-free (Ga,Al,In)N material on lattice mismatched materials which are removable to enable low defect density, large area, substantially crack-free, free-standing (Ga,Al,In)N material.
It is yet another object of the present invention to provide (Ga, Al, In)N material on lattice-mismatched materials such as sapphire, with or without buffer layers, surface preparation, nucleation layers, or other intermediate layers between the substrate and the (Ga,Al,In)N material.
It is another object of the invention to provide a two-stage growth technique that overcomes the deficiencies of the prior art.
Other objects and advantages will be more fully apparent from the ensuing disclosure and appended claims.